1. Field of the Invention
The present invention relates generally to an electric rotating machine mounted on a vehicle such as a passenger car or a truck, and more particularly to the electric rotating machine performing an operation for protecting each of rectifier modules disposed in the machine from overheating.
2. Description of Related Art
A control device for controlling an electric motor is well known (e.g., refer to Published Japanese Patent First Publication No. 2000-324893). This control device detects the temperature of a switching element formed in a semiconductor for electric power conversion by detecting the forward voltage of a diode of the switching element, and controls the switching element to be set in the off state when judging that the switching element is overheated.
Further, a control device for a synchronous power generator of a vehicle is well known (e.g., refer to Published Japanese Patent First Publication No. H09-19194). This control device detects the temperature of a field coil of the generator, and controls the field coil such that a field current is reduced when the detected temperature exceeds a predetermined value. Moreover, a control device for an alternator of a vehicle is well known (e.g., refer to Published Japanese Patent First Publication No. H08-336239). This control device detects the temperature of the alternator by detecting a field current of the alternator and controls the alternator such that the field current is reduced when the temperature of the alternator exceeds a predetermined threshold value. Furthermore, an automatic voltage adjusting device for an alternator is well known (e.g., refer to Published Japanese Patent First Publication No. H08-140400). This adjusting device uses a Zener diode having a function of temperature compensation, and the Zener diode produces a reference voltage depending on the temperature of the generator.
The prior art disclosed in the Publication No. 2000-324893 relates to the control of the electric motor. Therefore, this prior art cannot be applied for the control of a power generator used for a vehicle in which a current rectifying operation is performed by using switching elements formed of metal-oxide-semiconductor (MOS) transistors. More specifically, even when a control device controls a switching element formed of MOS transistors to be turned off, an electric current still flows through a parasitic diode formed around the transistors. Therefore, heat generated during the off state of the switching element becomes larger than heat generated during the on state of the switching element. This means that, even when the switching element is turned off to reduce heat excessively generated in the switching element, the control device cannot prevent the switching element from overheating caused by the parasitic diode.
In the prior art disclosed in the Publication No. H09-19194, the Publication No. H08-336239 and the Publication No. H08-140400, the field current is set based on the temperature of the field coil or the like. However, even when this setting method of the field current is used for a generator for a vehicle, the control device cannot prevent the rectifier from overheating. More specifically, although the detected temperature in the prior art disclosed in the above-described Publications generally relates with the temperature of the MOS transistors forming the rectifier, the detected temperature does not accurately indicate the temperature of the MOS transistors. Therefore, the control device cannot prevent the MOS transistors from overheating caused in the MOS transistors. Further, many MOS transistors are used for the rectifier of the generator for vehicle. Because functions of the MOS transistors in the rectifier are different from one another, it is impossible or very difficult to prevent the MOS transistors from being overheated, while considering different functions of the MOS transistors.